1. Field of the Invention
This invention relates generally to balanced to unbalanced transformers (baluns) and more particularly to an improved Marchand balun adapted for use on printed circuit boards.
2. Background Art
In radio frequency integrated circuits, it is often desirable for the input and output connections to be differential. An example of a differential connection is two wires having an equal impedance to a common ground conductor, their respective signals 180 degrees out of phase. A transmission line having these characteristics is known as a balanced line, as opposed to an unbalanced line. The advantages of a balanced radio frequency signal input over an unbalanced input include higher dynamic range, higher bandwidth, and lower pick-up and generation of interference.
Unfortunately, many radio frequency components, such as coaxial cable, are unbalanced. An adapter is required to convert the unbalanced signal into a balanced one without loss or distortion and while maintaining the proper matching impedance to terminate the transmission line. A passive device that achieves this function is known as a balun, and can be constructed in various ways. Many existing passive baluns either are too large, too expensive, too complex or have an insufficient bandwidth to be effectively implemented in a printed circuit board RF application.
What is needed is a passive balun that exhibits low signal distortion, high bandwidth, low loss, and has a good impedance match. Additionally, the passive balun should be small, simple to manufacture, and tolerant of variations.
The invention is a modified Marchand balun implemented using a plurality of first coupled printed metal traces electromagnetically coupled to a plurality of second coupled printed metal traces. The second coupled metal traces are also coupled to ground. Capacitors are coupled between the balun input and the first coupled metal traces, between ground and the first metal traces and between the second coupled printed metal traces and each of a pair of balun outputs.
The balun outputs a balanced signal, one output having an equal amplitude and opposite phase relative to a second output. Some of the metal traces comprising the balun are configured as transmission lines. These transmission lines are either coplanar waveguide transmission lines or microstrip transmission lines. A ground is located at the periphery or beneath the balun. Some embodiments place a ground at the periphery and below the balun. An impedance matching network is coupled to the input of the balun and an impedance matching network is coupled to the balanced output. Both the input and output impedance matching networks can comprise lumped or distributed element components.
A capacitor and spiral inductor can be coupled to the balun input. A direct current or voltage source and low frequency digital control signals can be applied to the balun input without electrically loading the balun at a desired input signal frequency. The applied direct current or voltage source can power various active circuits such as a low noise block via a coaxial cable.
The value of each lumped element, each distributed element and the width and spacing of the printed metal traces is incrementally varied in a simulator to determine the specific values that result in the desired balun circuit characteristics.